Temperature-dependent gas-phase 'H NMR spectra of six C-substituted TV,TV-dimethylamides (XCON(CH3)2) obtained at 500 MHz are consistent with the following free activation energies, AG*29i (kcal/mol): X = Cl, 15.4 (0.2); Br, 14.1 (0.2); N3, 16.5 (0.1); CN, 19.0 (0.1); CH2F, 15.2 (0.1); CHF2, 17.4 (0.1). For all of the amides studied, the observed first-order unimolecular rate constants are consistent with gas-phase <3*298 values for the internal rotation process which are 1-2 kcal/mol lower than reported condensed phase values and, in each case, consistent with a process proceeding via a transition state which has greater steric requirements than the equilibrium conformation.
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